Fabrice Tourre and the limits of Wall Street prosecutions

FORTUNE — Americans still frustrated that bankers have not been prosecuted for igniting the economic meltdown five years ago can take some solace with the sight of a young Wall Street trader in the dock this week.

When he worked for Goldman Sachs GS, Fabrice Tourre, according to the government, misled investors so they would buy mortgage securities that were created to fail. Tourre is an exception to the dearth of prosecutions against reckless bankers, but the public may rightly wonder why such a low-level employee has been singled out.

Tourre, 34, faces penalties that could include a significant fine or a lifetime ban from working in the securities industry. Although the civil prosecution, by the Securities and Exchange Commission, does not offer the emotionally satisfying ending of seeing him locked up, some experts say that it is the path best suited to police Wall Street.

The government “should use the regulatory tools that are available … if it is serious about not sending messages that the rich and powerful are immune from prosecution where the disenfranchised are targeted,” says Gregory Gilchrist, an associate professor at the University of Toledo College of Law. “This is an example of it working,” referring to Tourre’s courtroom ordeal.

Gilchrist notes that agencies already have the power to impose serious civil penalties, including fines and industry employment bans that “will deter and send a clear message.”

Pinning the blame for the economic crisis has been vexing for the government, which has brought only a small number of cases against individuals. Tourre is accused of acting as an architect of Goldman’s risky mortgage securities program, called Abacus. In 2010, the firm paid $550 million to settle the government’s charges.

So far, only relatively small-fry Wall Streeters — albeit very well compensated — have seen the inside of a courtroom, and settlements without any admission of guilt have been the norm.

Regulators have also skipped past high-level firm officials who set and controlled policy at Goldman Sachs. Tourre had sent frank personal emails boasting of selling such mortgage securities to widows and orphans. But bragging is no assurance that he will be found liable.

That’s because culpability is hard to prove. “When you get into the spongier areas of getting a bank to explain its business judgment, it’s harder to make a case,” explains Lawrence G. Baxter, a professor at Duke University Law School who specializes in financial services regulation.

“When there is a clearly defined crime, convictions are easier to get,” he notes. He says the case in the U.K. against financial workers that allegedly tampered with Libor, a rate used to determine the cost of lending between banks, may be easier to prove because it involves information that appears to confirm coordination among different individuals to depress the rate.

At best, however, successful prosecutions are more difficult than ever to bring, says Jonathan Macey, a Yale Law School professor who focuses on corporate and securities law, and the author of The Death of Reputation, because “individuals are completely separate now from the institutions where they work.

“It used to be that getting sued by the SEC for fraud or insider trading was stigmatizing — like child abuse charges — but that no longer applies.”

Regulators are also judged by different standards today. The public and Congress measure the securities agency’s success, Macey says, “by how many cases they bring and how much money they collect in fines. It is much easier to settle 20 cases for the same time and money it takes to bring one difficult case.”

So, is there a solution? Gilchrist say banks can be an elusive target in large part because they are more interconnected than corporations, and because of fears that prosecution could destabilize them and hurt customer deposits. Banks may be “large, systemically important, and fragile in a way that counsel against prosecution, but bank employees are not,” he adds.

Another complication, he notes, is that criminal charges must be proved beyond a reasonable doubt, which is hard to achieve. But regulatory agencies have plenty of tools, he says, pointing to fines and bans that “can be imposed for mere recklessness proven by a preponderance of the evidence.”

The prospect “of losing one’s income and bonuses from the period during which crimes within the bank continued — if made real through enforcement — would provide a powerful incentive to provide oversight, maintain compliance, and terminate potentially criminal practices,” Gilchrist argues in a forthcoming article in the University of Colorado Law Review.

The clock is ticking on punishing Wall Street wrongdoers for conduct that occurred five years ago, or more, and Tourre may wind up being little more than a footnote in the post-recession saga.

Going forward, the SEC could employ a little-used legal tactic that it has used to hold corporate executives accountable for wrongdoing. The approach — called control person liability — makes executives like division heads accountable for what goes on in their fiefdoms. This would end government inquiries laboriously looking around a financial firm for evidence to tag wrongdoers, and make it clear-cut that the executive in charge of questionable activities would be responsible.

Holding bankers accountable “would send a powerful message to bankers,” says Duke University’s Baxter. “Regulators say it’s hard to identify the wrongdoers, but I know from my experience working at a bank that examiners know the people they supervise every day.”

But the conundrum comes down to more than just cozy workers, he says. “The problem is our own ambivalence about banks and the role they play.”